The present invention relates to a liquid dispensing system and packaging apparatus which includes such a system. It is particularly concerned with a system for dispensing liquid gas by which is meant a gas in liquid form and which liquid vapourises at a temperature less than zero degress centigrade at atmospheric pressure. Typical examples of liquid gases are nitrogen, oxygen and argon. These liquid gases, sometimes referred to as cryogenic liquids, are widely used in industry, inter alia for the pressurising and/or purging of foodstuff or beverage packages or containers.
A conventional system for dispensing liquid gas (as above defined) is known in the art as the Meser Griesheim system in which a reservoir of liquid gas is maintained, substantially at a constant depth, in an insulated vessel having an outlet port through which a constant stream of the liquid gas emerges under gravity; the liquid gas stream is directed downwardly into the open tops of packages which are fed successively beneath the outlet port. The dose of liquid gas which each package receives is primarily determined by the head of liquid gas in the reservoir, the dimensions of the outlet port and the speed at which the package passes on a conveyor beneath the outlet port; the intention is that these characteristics are adjusted to provide a predetermined volume of liquid gas in each package so that such liquid gas will vapourise to purge the package of air and/or provide a predetermined pressure within the package when the latter is sealed (which usually occurs immediately following the dosing of the package with the liquid gas).
A particular problem encountered with the aforementioned Griesheim system is that although the vessel for the liquid gas is insulated, there is a tendency for the outlet port to become blocked with ice build-up as water droplets in the atmosphere freeze around the outlet port. A similar problem can occur when liquid gas is dosed into packages which contain liquid and liquid droplets caused by back-splashing from the package can freeze on and around the outlet port. Clearly such freezing has a detrimental effect upon the dosages which are applied to the packages/containers. In an attempt to alleviate this ice build-up the container vessel of the liquid gas reservoir is provided with an electrical heater in the vicinty of the outlet port with the intention that the ice can be melted off as required. However, in practice and even with an electrical heater, it is found that the outlet port becomes blocked with ice and it is necessary to halt the package conveyor and dispensing system, empty it of liquid gas and clean off the outlet port and adjacent areas with the resultant expense and inconvenience.
A further disadvantage of the Griesheim system is that the continuous stream of liquid gas that is dispensed usually results in considerable quantities of it being wasted, especially where the open topped packages pass beneath the outlet port successively in a spaced array. Further, it is often found that there are inconsistencies in the dosages of liquid gas which are applied to the open topped containers, for example as a result of the outlet port becoming restricted or blocked as aforementioned or by the speed of the conveyor for the open topped packages changing inadvertently.
It has been proposed to modify the Griesheim system, for example as disclosed in British patent specification No. 2,215,446A, to include a reciprocating valve member in the reservoir which opens and closes the outlet port. The valve member is controlled to interrupt the stream of liquid gas which emerges from the outlet port so that doses of liquid gas are dispensed successively and these can be synchronised with open topped packages passing beneath the outlet port. Although this alleviates wastage of the liquid gas as compared with the constant stream dispensing system, the gravity feed of the liquid gas through the outlet port is susceptible to changes in the depth or head of the liquid gas in the reservoir and as a result of changes in this depth the doses of liquid gas which are dispensed frequently vary in volume to an unacceptable extent. Furthermore, the gravity feed of the liquid gas through the outlet port restricts the speed at which successive doses can be dispensed, thereby restricting the rate at which the doses can be introduced successively into the open topped packages (so that system is generally regarded as being appropriate for low speed packaging only).
It has also been proposed, for example in British patent specification A-392,655, to dispense liquid through an outlet port of a reservoir by use of a reciprocating piston and cylinder device in the reservoir. In this proposal the outlet port is provided with a spring loaded non-return valve which closes the outlet port as the piston moves to expand a chamber in its cylinder and draw liquid into that chamber from the reservoir and which opens under pressure from the liquid in the chamber as that chamber is contracted by movement of the piston to eject a dose of liquid through the outlet port. Although this proposal has the advantage that the reciprocating piston may provide successive liquid doses at high speed it is quite unsuitable for use in dispensing cryogenic liquids or liquid gas. In use with liquid gas the non-return valve in the outlet port would rapidly seize or become frozen at the low temperatures involved and thus require frequent servicing which would be unacceptable in a high speed packaging system.
It is an object of the present invention to provide a cryogenic liquid or liquid gas dispensing system and by which the problems of known liquid dispensing systems and as discussed above may be alleviated.